The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or triangular straight-through openings or cells with catalyst deposted on the walls of the cells, catalyst coated refractory metal oxide beads, e.g., alumina beads, or a corrugated thin metal foil monolith, e.g., ferritic stainless steel foil, having catalyst carried on or supported by the surface The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere. However, conversion is not efficient initially when the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the exhaust gases come in contact must be at a minimum temperature, e.g., 390.degree. F. for carbon monoxide, 570.degree. F. for voltile organic compounds (VOC) and 1000.degree. F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited (diesel) and spark ignited engines have this need.
To achieve initial heating of the catalyst prior to engine start-up, there is provided an electrically heatable catalytic converter formed of a corrugated thin metal foil monolith which is connected to a voltage source, e.g., a 12 volt or 14 volt automotive battery, and power supplied, preferably before and during start-up, to elevate and maintain the temperature of the catalyst to at least about 650.degree. F.
Copending application Ser. No. 587,219 filed 24 Sep. 1990 (and its parent case Ser. No. 524,284 filed 16 Apr. 1990, now abandoned) discloses one form of electrically heatable catalytic converter which has been found to be subject to telescoping of the core, and provides one means for offsetting the tendency to telescoping of the core in operation and ultimate destruction thereof in the face of "Hot Shake" automotive proof tests . The present invention provides a different means for offsetting the tendency to telescope. Because much of the disclosure of Ser. No. 587,219 is relevant to the present application, the disclosure of Ser. No. 587,219 is incorporated herein by reference thereto. Instead of utilizing a ceramic core in juxtaposition with the electrically heatable catalytic core to inhibit telescoping, the present invention utilizes inter-leaf brazing whereby the leaves are held against telescoping or axial displacement. Ser. Nos. 524,284 and 587,219 are commonly owned with the present application.
Copending application Ser. No. 606,130 filed 31 Oct. 1990, now U.S. Pat. No. 5,070,694 by William A. Whittenberger and entitled Structure for Electrically Heatable Catalytic Converter, and commonly owned with the present application, discloses a means for preventing telescoping of a spiral or S-wound corrugated thin metal foil monolith by brazing between the corrugated leaves according to a pattern (staggered relation). In that application, all of the leaves forming the monolith are corrugated. The adjacent leaves or strips are in a nonnesting relationship by virtue of (1) pattern corrugations, such as herringbone corrugation, or (2) straight-through corrugations using corrugated leaves with straight cells of differing pitch. (See U.S. Pat. No. 4,810,588 dated 7 Mar. 1989 to Bullock)
The present application is quite similar to the aforesaid U.S. Pat. No. 5,070,694, except that the monolith is made up of both corrugated and flat thin stainless steel strips. Nesting is not a problem with a structure in which corrugated foil strips are alternated with flat strips. However, telescoping of the core under the severe conditions of the "Hot Shake" test described below is still a problem.
Reference may also be had to U.S. Pat. No. 4,381,590 dated 3 May 1983 to Nonnenmann et al which discloses a spirally wound monolith made up of corrugated and flat continuous strips which are brazed together. The present invention utilizes a plurality of strips, corrugated alternating with flat, all emanating from a central core and having a length so as to get appropriate wattage (on a 12 volt battery voltage source) between the central core and the outer shell. Generally, this power level ranges from about 1500 watts to about 12000 watts. Reference may be had to Ser. No. 587,219, supra, Tables I and II.
The electrically heatable catalytic cores hereof are normally spirally wound, or S-wound, corrugated and flat thin metal foil strips in alternating laminar relation. The corrugated and flat thin metal foil strips are not initially washcoated with a refractory metal oxide coating, e.g., an alumina coating, and a catalyst. The latter treatment steps come later according to the present invention for reasons which will appear. The corrugated and flat thin metal cores are subjected to a severe test which they must pass in order to be acceptable for automotive use. This test (so called "Hot Shake Test") involves oscillating (100-200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at high temperature (between 700.degree. and 950.degree. C., 1292.degree. F. and 1742.degree. F., respectively) with exhaust gas from a running internal combustion engine being passed through the device. If the electrically heatable catalytic device telescopes in the direction of gas flow or breaks up after a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually, the test device will fail in 5 hours if it is going to fail. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
Accordingly, it is a principal object of &he present invention to provide a device which will pass the foregoing test and thereby prove reliable in extreme field service.
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated 30 Oct. 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated 30 Oct. 1990 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one corrugated, the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. Heating of the ceramic core by conduction takes too long to solve the problem of atmospheric pollution at start-up. These cores are, moreover, subject to telescoping under the conditions of the foregoing severe test.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated 8 Dec. 1987 for details of a process for corrugating thin metal foil strips. The coating of the surface with a refractory metal oxide is not done in the present case or the application of the catalyst as described in that patent. These steps cannot be performed in the present case as the resulting surfaces cannot be brazed. Also, for brazing purposes, it is desirable not to stress relieve aluminum-containing stainless steels as there is a tendency to form oxides on the surface to which the brazing does not adhere well. However, the corrugating of a thin metal foil strip with a herringbone, or chevron pattern as taught therein is applicable to the present application and to that extent, the disclosure of the aforesaid U.S. Pat. No. 4,711,009 is incorporated herein by reference. The refractory metal oxide coating and the catalyst are applied by dipping in the present process after formation of the core. The composition of the washcoat and the catalyst treating solutions as taught in U.S. Pat. No. 4,711,009 are pertinent to the present process.
Reference may also be had to International PCT publication numbers WO 89/10471 and WO 10470 each filed 2 Nov. 1989. S-wound cores composed of corrugated and flat strips in alternating relation are disclosed in these publications. However, there is no teaching of brazing between the corrugated thin metal foil layers, and telescoping of the core under the conditions of the severe test described above occurs.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen et al. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002%to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium and praseodymium, or a mixture of two or more thereof, balance iron and steel making impurities.
In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 dated 5 Mar. 1974 to Sowman for formulations and manufacture of ceramic fibers and mats useful herein. One such ceramic fiber material is currently commercially available from 3-M under the registered trademark "INTERAM."